- Email: [email protected]
Vancomycin-resistant Staphylococcus aureus: no apocalypse now
EDITORIAL Vancomycin-resistant Staphylococcus aureus: no apocalypse now F. W. Goldstein and M. D. Kitzis Hospital Saint Joseph, Paris, France The number of reports concerning vancomycin-resistant Staphylococcus aureus is much higher than the number of true resistant strains or unexpected clinical failures. Many confounding factors, including inadequate serum levels, severely ill patients, foreign devices or undrained abscesses, are more likely to be responsible for the clinical failures than resistance to vancomycin. Keywords GISA, VRSA, vancomycin resistance, Staphylococcus aureus Accepted 14 January 2003
Clin Microbiol Infect 2003; 9: 761±765 `Vancomycin-resistant Staphylococcus aureus: apocalypse now?' was the title of a paper [1] published a few months after Hiramatsu et al. [2] announced the discovery of a vancomycin-resistant strain: unprecedented agitation followed this publication. A few years later, the question was: `Intermediate vancomycin resistance in Staphylococcus aureus: a major threat or a minor inconvenience?' [3]. Finally, two years on, the question became: `Glycopeptide-intermediate Staphylococcus aureus: rediscovery of an old problem?' [4]. During the last four years, dozens of papers have been published worldwide, describing the occurrence of glycopeptide-intermediate S. aureus (GISA), in some cases isolated more than ten years ago, and known as VISA, VRSA, hetero-VISA, hetero-VRSA, or hetero-GISA [5±17]. As described below, all these strains have increased MICs for all glycopeptides and exhibit heterogeneous behavior on glycopeptide-containing agar. They will simply be referred to here as GISA. There are probably not more than ®ve or six strains in the world with MICs of 8 mg/L for vancomycin, similar to Mu 50 (tested under standard conditions). All the other strains are hetero-GISA like Mu 3, with MICs not exceeding 3 mg/L. This is also true for the strains that we have described and isolated over the last ten years in our hospital [18,19]. Corresponding author and reprint requests: F. W. Goldstein, Laboratoire de Microbilogie MeÂdicale, Hospital Saint Joseph, 185 rue Raymond Losserand, F-75014 Paris, France Tel: 33 44 12 3637 Fax: 33 1 44 12 36 85 E-mail: [email protected]
What has changed since 1997? There has been neither the dissemination of a clone, nor the emergence of GISA in different countries. The modi®cation of MIC determination procedures or interpretation and the detection of the heterogeneous expression of this resistance are responsible for the worldwide `emergence' of such strains [2,20], very similar to strains reported many years ago and selected in vivo or in vitro by teicoplanin and considered as susceptible to vancomycin [18,19,21±25]. Better detection of GISA will clearly help the clinician to better manage infections due to such strains. However, the most important question is: are GISA really responsible for a signi®cant increase in vancomycin failures? All GISA have a thick cell wall responsible for the ampli®cation of the naturally occurring af®nity-trapping of vancomycin by unprocessed D-ala D-ala. These bacteria have undergone numerous small adjustments of cell wall metabolism without the presence of acquired foreign elements [26,27]. This explains why, in contrast to other mechanisms of resistance, there is no speci®c genetic marker and no molecular method able to detect such strains. GISA have a high ®tness cost: they have a prolonged generation rate, growing as small colonies, need large amounts of nutrients, particularly glutamine (and glucose for some strains), and cannot prevail without selective pressure. They are selected by long-term glycopeptide usage and, much more surprisingly, by b-lactams [26]. GISA can also be selected in vivo by as yet undetermined factors in the absence of any antibiotic
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases
762 Clinical Microbiology and Infection, Volume 9 Number 8, August 2003 selective pressure [8]. In the absence of this pressure, GISA may become fully susceptible when grown in vitro, or retain their heterogeneous nature when population analysis pro®les (PAPs) are performed [20]. Resistant cells produce altered murein monomers (containing glutamate instead of glutamine), which have a higher af®nity for vancomycin, increasing its trapping, and are poorer substrates for penicillin binding protein (PBP) transpeptidases. Moreover, there is an increase in murein monomer synthesis or a decrease in cell wall turnover, resulting in increased thickening of the cell wall: GISA have 30±40 peptidoglycan layers, as compared to only 20 for normal S. aureus [20,26]. As a result, more vancomycin is trapped before reaching the cytoplasmic membrane and, in addition, this trapping is responsible for a `clogging' phenomenon by steric hindrance due to the size of the molecule. An increase in both PBP2 and PBP2a is observed in all GISA, as we also reported seven years ago [18,27]. As vancomycin and PBPs are able to block binding by the other molecule to the terminal D-ala D-ala of the peptidoglycan precursor, an increase in PBP2 or PBP2a is associated with an increase in the amount of vancomycin necessary to block the binding of PBP to its target. On the other hand, there is a decrease in the amount of PBP4, responsible for a decrease in cross-binding, and an increased amount of unprocessed D-ala D-ala; the loss of PBP activity is directly related to increased vancomycin MICs. Other mechanisms of resistance, however, cannot be excluded. The most important discovery of Hiramatsu [20] is the detection of the heterogeneous expression of GISA strains by PAP. This clearly discriminates GISA from normal S. aureus: at a concentration of 3±4 mg/L, all the cells of normal S. aureus are inhibited, even with a high inoculum of 107 108 CFU [2,20]. In contrast, all the GISA will yield at least 1000 colonies, sometimes many more. The number of colonies will decrease with increased vancomycin concentrations, up to 10± 32 mg/L. As suggested by Hiramatsu, a `onepoint PAP', which is obtained by screening a high inoculum on plates containing 4±6 mg/L of vancomycin, clearly discriminates between GISA and normal S. aureus. Because of the poor reproducibility of the pro®les in different laboratories, the poor stability of the resistance, and the lack of
standardized procedures, PAP remains the only simple and reliable technique for detecting GISA strains. The PAP also very clearly explains the relationship between the inoculum size and the MIC, which depends on the heterogeneity of the strain. With a light inoculum of 102 103 CFU/spot, as often performed on agar, the MICs will decrease to fully susceptible levels. The main question is: are GISA really responsible for an increased clinical failure rate? If so, are there other therapeutic options? Failure of vancomycin in the treatment of staphylococcal infections has been observed and reported over the last 40 years. As with other antibiotics, these failures can be explained not only by decreased susceptibility, but also by low antibiotic serum levels, undrained abscesses, or the presence of foreign bodies. All the failures reported so far for vancomycin in the treatment of GISA infections were `expected failures', in that there were several `good' reasons for them, as reported for many other antibiotics, in addition to vancomycin: 1. Inadequate vancomycin levels or no monitoring. 2. Use of teicoplanin, which is less active, or no monitoring. 3. Antagonistic combinations with aminoglycosides and/or b-lactams. 4. No drainage of large pus collections. 5. Foreign devices not removed. 6. Very severely ill patients. In some patients, several explanations were relevant. 1. What is an adequate serum level for vancomycin? In a paper published in 1993, it was considered that at least 43% of patients treated with vancomycin had inappropriate drug monitoring. It was a very optimistic approach, because `acceptable serum levels were a trough of 5±10 mg/mL and a peak of 20±25 mg/mL'. In contrast to many papers suggesting that it is not necessary to perform vancomycin serum assays, we perform these routinely for all severely ill patients. Patients are treated according to well established nomograms with the standard dose of 1 g twice daily or by continuous infusion after a 1-g loading dose. Vancomycin has a very variable half-life, 3±12 h in healthy volunteers, and there is no reason for it
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 9, 761±765
Editorial 763
to have a smaller range in severely ill patients. Assuming 50% protein binding in serum and increased non-speci®c `af®nity trapping' by the staphylococcus, the effective serum concentration above the MIC for 40±50% of the dosing interval or, better, above the concentration inhibiting 100% of cells with a high inoculum (as determined by PAP) should be at least three times this concentration: (a) 9±12 mg/L for a fully susceptible strain (MIC: 1±2 mg/L with 100% inhibition at 3±4 mg/L). (b) 30 mg/L for a hetero-GISA type Mu 3 (MIC 2± 3 mg/L with 100% inhibition at 10 mg/L). (c) At least 40 mg/L for Mu 50-like strains, which is more dif®cult to maintain safely, and without exceeding 50 mg/L. This may represent 4± 5 g/daily administered as a continuous infusion (N. Desplaces, personal communication). It should be pointed out that puri®ed vancomycin is very well tolerated and has little resemblance to the historical `Mississipi mud' used 45 years ago. According to the results obtained in 553 patients in our hospital receiving vancomycin on the basis of published nomograms, we consider that only 6.5% and 23% of patients receiving vancomycin either as a standard 1-g treatment twice daily or continuous infusion (after a 1-g loading dose), respectively, had adequate levels for the treatment of Mu 3-type GISA strains; more than 40% of patients treated with this standard dose had inadequate levels for treatment of even fully susceptible strains. Among all the published failures, only one patient [12] had adequate serum levels; the others occasionally had excessively low trough and/or peak serum levels. In conclusion, the only thing as dangerous as an overdose is an underdose. 2. Teicoplanin selects ®rst-step mutants or mutants with higher MICs much more easily and has been associated with clinical failures. Moreover, with 90% protein binding and in the absence of a loading dose for three days and monitoring, effective serum levels will remain very low, as observed in one patient [14]. In many other patients [13,14,17], teicoplanin, and not vancomycin, has eventually selected GISA. 3. Vancomycin or teicoplanin has often been combined with an aminoglycoside for gentamicin-resistant strains [2,10,11,14,17]. We and many
others have reported for more than 20 years that gentamicin-resistant S. aureus produces the bifunctional enzyme aminoglycoside phosphotransferase (APH) (200 ) aminoglycoside acetyltransferase (AAC) (60 ) which inactivates all aminoglycosides (not streptomycin, an aminocyclitol). As a consequence, all combinations with b-lactams or a glycopeptide are antagonistic. The already weak bactericidal activity of vancomycin will be reduced. In respect of the combinations of vancomycin with b-lactams, some authors have reported that this combination is synergistic [6,9,14]. This is true for some b-lactam concentrations, or rather when the combinations are improperly tested. As already suggested by Hiramatsu, with the exception of Mu 50, combinations of b-lactams with vancomycin are antagonistic, at least over a range of concentrations. This was the case for a few patients [10,12]. 4. Because of the non-speci®c af®nity trapping, vancomycin is less effective in the presence of a high inoculum; this is particularly true for GISA, and this was the reason for the failures in patients [2,12,13,17] who had undrained abscesses. For the same reasons, some patients were cured after drainage [13,17], whereas others died [12,13]. 5. As with other antibiotics, the presence of catheters and other foreign devices strongly reduces the activity of vancomycin, especially against GISA, which have a thicker cell wall and increased attachment to arti®cial devices. These devices should be removed as soon as possible. Removal of catheters has undoubtedly contributed to the recovery of many patients [13,17,28]. The presence of a catheter for a long period is responsible for thrombophlebitis and recurrent breakthrough bacteremia; this was the reason for failure in several patients [9±11,15,17]. 6. One patient died within 12 h of admission; no antibiotic could have helped him [10]. Another patient died because he refused surgery [29]: it is well known that about 50% of leftsided staphylococcal endocarditis patients will die in the absence of surgery. Several patients were very severely ill, old and at a late stage of a progressive disease; it is very unlikely that other antibiotics would have been of any help [9,14,15,29]. It has not yet been demonstrated that vancomycin, administered at high dosage and properly
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 9, 761±765
764 Clinical Microbiology and Infection, Volume 9 Number 8, August 2003 monitored, is not effective against GISA, at least of the Mu 3 type. Assuming than vancomycin alone would not be effective, what are the other issues? 1. Vancomycin in combination with b-lactams: these associations are antagonistic as suggested by Hiramatsu, b-lactams may increase the MIC of vancomycin or select for heterogeneous strains [20]. 2. Vancomycin in combination with other antibiotics. The association of vancomycin and aminoglycosides is antagonistic for all staphylococci producing aminoglycoside-inactivating enzymes. Against susceptible bacteria, the association is highly bactericidal with other antibiotics (quinupristin±dalfopristin, rifampin, fusidic acid, cotrimoxazole), the association is indifferent or displays an antagonistic effect for a particular concentration range. (R. Atoui, unpublished data). 3. Other antibiotics without glycopeptides: most of the GISA are susceptible to linezolid, quinupristin±dalfopristin, chloramphenicol or co-trimoxazole, and occasionally to tetracyclines, fusidic acid or rifampin. These antibiotics have been successfully used for the treatment of a few patients [11,14,17]. 4. Investigational compounds: tigecycline, oritavancin or daptomycin have not yet been evaluated in vivo. Finally, all the new ¯uoroquinolones have poor activity against these multiresistant bacteria and should not be used. There are no well documented clinical studies assessing the role of decreased vancomycin susceptibility as the sole or main factor in therapeutic failure. In all the reported failures, there have been several important confounding factors explaining a breakthrough infection. Obviously, however, there is no reason for GISA to give better clinical results than fully susceptible strains or to cure the patient of a fatal underlying disease. It is dif®cult to predict the future. Once staphylococci acquire resistance to older and newer (linezolid, quinupristin±dalfopristin) antibiotics in addition to true high-level resistance to vancomycin (vanA), as just described during the preparation of this manuscript [30], then `apocalypse now' may become true. REFERENCES 1. Tabaqchali S. Vancomycin-resistant Staphylococcus aureus: apocalypse now ? Lancet 1997; 350: 1644±45.
2. Hiramatsu KN, Aritaka H, Hanaki S et al. Dissemination in Japanese hospitals of strains of Staphylococcus aureus heterogeneously resistant to vancomycin. Lancet 1997; 350: 1670±3. 3. Johnson AP. Intermediate vancomycin resistance in Staphylococcus aureus: a major threat or a minor inconvenience? J Antimicrob Chemother 1998; 42: 289±91. 4. Cercenado E. Glycopeptide-intermediate Staphylococcus aureus: rediscovery of an old problem? Clin Microbiol Infect 2000; 6: 517±18. 5. Chesneau O, Morvan A, El Sohl N. Retrospective screening for heterogeneous vancomycin resistance in diverse Staphylococcus aureus clones disseminated in French hospitals. J Antimicrob Chemother 2000; 45: 887±90. 6. Climo MW, Patron RL, Archer GL. Combinations of vancomycin and b-lactams are synergistic against staphylococci with reduced susceptibilities to vancomycin. Antimicrob Agents Chemother 1999; 43(7): 1747±53. 7. Guerin F, Buu-HoõÈ A, Mainardi JL et al. Outbreak of methicillin-resistant Staphylococcus aureus with reduced susceptibility to glycopeptides in a Parisian hospital. J Clin Microbiol 2000; 38(8): 2985±8. 8. Vaudaux P, FrancËois P, Berger-BaÈchi B, Lew DP. In vivo emergence of subpopulations expressing teicoplanin or vancomycin resistance phenotypes in a glycopeptide-susceptible, methicillin-resistant strain of Staphylococcus aureus. J Antimicrob Chemother 2001; 47(2): 163±70. 9. Howe RA, Wootton M, Bennett PM, MacGowan AP, Walsh TR. Interactions between methicillin and vancomycin in methicillin-resistant Staphylococcus aureus strains displaying different phenotypes of vancomycin susceptibility. J Clin Microbiol 1999; 3068±71. 10. Rotun SS, McMath V, Schoonmaker DJ et al. Staphylococcus aureus with reduced susceptibility to vancomycin isolated from a patient with fatal bacteremia. Emerg Infect Dis 1999; 5: 147±9. 11. Smith TL, Pearson ML, Wilcox KR et al. Emergence of vancomycin resistance in Staphylococcus aureus. N Engl J Med 1999; 340: 493±01. 12. Sugino Y, Iinuma Y, Ichiyama S et al. In vivo development of decreased susceptibility to vancomycin in clinical isolates of methicillin-resistant Staphylococcus aureus. Diagn Microbiol Infect Dis 2000; 38: 159±67. 13. Trakulsomboon S, Danchaivijitr S, Rongrungruang Y et al. First report of methicillin-resistant Staphylococcus aureus with reduced susceptibility to vancomycin in Thailand. J Clin Microbiol 2001; 39(2): 591±5. 14. Ward PB, Johnson PDR, Brabsch EA, Mayall BC, Grayson ML. Treatment failure due to methicillin-resistant Staphylococcus aureus (MRSA) with
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15.
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reduced susceptibility to vancomycin. MJA 2001; 175: 480±3. Samson SY, Wong PL, Ho PC, Woo Y, Yuen KY. Bacteremia caused by staphylococci with inducible vancomycin heteroresistance. Clin Infect Dis 1999; 29: 760±7. Rybak MJ, Akins RL. Emergence of methicillinresistant Staphylococcus aureus with intermediate glycopeptide resistance: clinical significance and treatment options. Drugs 2001; 61(1): 1±7. Ploy MC, GreÂlaud C, Martin C, de Lumley L, Denis F. First clinical isolate of vancomycin-intermediate Staphylococcus aureus in a French hospital. Lancet 1998; 351: 1212. Mainardi JL, Shlaes DM, Goering RV, Shlaes JH, Acar JF, Goldstein FW. Decreased teicoplanin susceptibility of methicillin-resistant strains of Staphylococcus aureus. J Infect Dis 1995; 171: 1646±50. Quincampoix JC, Carbonne A, Loyer D et al. Follow-up of a nosocomial outbreak of methicillin-resistant strains of Staphylococcus aureus (MRSA) with decreased susceptibility to teicoplanin (DST) [abstract C167]. In: 37th ICAAC, Toronto. 1997. Hiramatsu K. Vancomycin-resistant Staphylococcus aureus: a new-model of antibiotic resistance. Lancet Infect Dis 2001; 1: 147±55. Biavasco F, Giovanetti E, Montanari MP, Lupidi R, Varaldo PE. Development of in-vitro resistance to glycopeptide antibiotics: assessment in staphylococci of different species. J Antimicrob Chemother 1991; 27: 71±9. Brunet F, Vedel G, Dreyfus F et al. Failure of teicoplanin therapy in two neutropenic patients with staphylococcal septicemia who recovered after administration of vancomycin. Eur J Clin Microbiol Infect Dis 1990; 9: 145±7.
23. Daum RS, Gupta S, Sabbagh R, Milewski WM. Characterization of Staphylococcus aureus isolates with decreased susceptibility to vancomycin and teicoplanin: isolation and purification of a constitutively produced protein associated with decreased susceptibility. J Infect Dis 1992; 166: 1066±72. 24. Kaatz G, Seo SM, Dorman NJ, Lerner SA. Emergence of teicoplanin resistance during therapy of Staphylococcus aureus endocarditis. J Infect Dis 1990; 162: 103±8. 25. Manquat G, Croize J, Stahl JP, Meyran M, Hirtz P, Micoud M. Failure of teicoplanin treatment associated with an increase in MIC during therapy of Staphylococcus aureus septicaemia. J Antimicrob Chemother 1992; 29: 731±2. 26. Cui L, Murakami H, Kuwahara-Arai K, Hanaki H, Hiramatsu K. Contribution of a thickened cell wall and its glutamine nonamidated component to the vancomycin resistance expressed by Staphylococcus aureus Mu50. Antimicrob Agents Chemother 2000; 44(9): 2276±85. 27. Shlaes DM, Shlaes JH, Vincent S, Etter L, Fey PD, Goering RV. Teicoplanin-resistant Staphylococcus aureus expresses a novel membrane protein and increases expression of penicillin-binding protein 2 complex. Antimicrob Agents Chemother 1993; 37: 2432±7. 28. Ariza J, Pujol M, Cabo J et al. Vancomycin in surgical infections due to methicillin-resistant Staphylococcus aureus with heterogeneous resistance to vancomycin. Lancet 1999; 353(9164): 1587±8. 29. Fridkin SK. Vancomycin-intermediate and -resistant Staphylococcus aureus: what the infectious disease specialist needs to know. Clin Infect Dis 2001; 32: 108±15. 30. Vancomycin-resistant Staphylococcus aureus. Pennsylvania 2002. MMWR 2002; 51: 902.
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 9, 761±765
Clin Microbiol Infect 2003; 9: 761±765 `Vancomycin-resistant Staphylococcus aureus: apocalypse now?' was the title of a paper [1] published a few months after Hiramatsu et al. [2] announced the discovery of a vancomycin-resistant strain: unprecedented agitation followed this publication. A few years later, the question was: `Intermediate vancomycin resistance in Staphylococcus aureus: a major threat or a minor inconvenience?' [3]. Finally, two years on, the question became: `Glycopeptide-intermediate Staphylococcus aureus: rediscovery of an old problem?' [4]. During the last four years, dozens of papers have been published worldwide, describing the occurrence of glycopeptide-intermediate S. aureus (GISA), in some cases isolated more than ten years ago, and known as VISA, VRSA, hetero-VISA, hetero-VRSA, or hetero-GISA [5±17]. As described below, all these strains have increased MICs for all glycopeptides and exhibit heterogeneous behavior on glycopeptide-containing agar. They will simply be referred to here as GISA. There are probably not more than ®ve or six strains in the world with MICs of 8 mg/L for vancomycin, similar to Mu 50 (tested under standard conditions). All the other strains are hetero-GISA like Mu 3, with MICs not exceeding 3 mg/L. This is also true for the strains that we have described and isolated over the last ten years in our hospital [18,19]. Corresponding author and reprint requests: F. W. Goldstein, Laboratoire de Microbilogie MeÂdicale, Hospital Saint Joseph, 185 rue Raymond Losserand, F-75014 Paris, France Tel: 33 44 12 3637 Fax: 33 1 44 12 36 85 E-mail: [email protected]
What has changed since 1997? There has been neither the dissemination of a clone, nor the emergence of GISA in different countries. The modi®cation of MIC determination procedures or interpretation and the detection of the heterogeneous expression of this resistance are responsible for the worldwide `emergence' of such strains [2,20], very similar to strains reported many years ago and selected in vivo or in vitro by teicoplanin and considered as susceptible to vancomycin [18,19,21±25]. Better detection of GISA will clearly help the clinician to better manage infections due to such strains. However, the most important question is: are GISA really responsible for a signi®cant increase in vancomycin failures? All GISA have a thick cell wall responsible for the ampli®cation of the naturally occurring af®nity-trapping of vancomycin by unprocessed D-ala D-ala. These bacteria have undergone numerous small adjustments of cell wall metabolism without the presence of acquired foreign elements [26,27]. This explains why, in contrast to other mechanisms of resistance, there is no speci®c genetic marker and no molecular method able to detect such strains. GISA have a high ®tness cost: they have a prolonged generation rate, growing as small colonies, need large amounts of nutrients, particularly glutamine (and glucose for some strains), and cannot prevail without selective pressure. They are selected by long-term glycopeptide usage and, much more surprisingly, by b-lactams [26]. GISA can also be selected in vivo by as yet undetermined factors in the absence of any antibiotic
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases
762 Clinical Microbiology and Infection, Volume 9 Number 8, August 2003 selective pressure [8]. In the absence of this pressure, GISA may become fully susceptible when grown in vitro, or retain their heterogeneous nature when population analysis pro®les (PAPs) are performed [20]. Resistant cells produce altered murein monomers (containing glutamate instead of glutamine), which have a higher af®nity for vancomycin, increasing its trapping, and are poorer substrates for penicillin binding protein (PBP) transpeptidases. Moreover, there is an increase in murein monomer synthesis or a decrease in cell wall turnover, resulting in increased thickening of the cell wall: GISA have 30±40 peptidoglycan layers, as compared to only 20 for normal S. aureus [20,26]. As a result, more vancomycin is trapped before reaching the cytoplasmic membrane and, in addition, this trapping is responsible for a `clogging' phenomenon by steric hindrance due to the size of the molecule. An increase in both PBP2 and PBP2a is observed in all GISA, as we also reported seven years ago [18,27]. As vancomycin and PBPs are able to block binding by the other molecule to the terminal D-ala D-ala of the peptidoglycan precursor, an increase in PBP2 or PBP2a is associated with an increase in the amount of vancomycin necessary to block the binding of PBP to its target. On the other hand, there is a decrease in the amount of PBP4, responsible for a decrease in cross-binding, and an increased amount of unprocessed D-ala D-ala; the loss of PBP activity is directly related to increased vancomycin MICs. Other mechanisms of resistance, however, cannot be excluded. The most important discovery of Hiramatsu [20] is the detection of the heterogeneous expression of GISA strains by PAP. This clearly discriminates GISA from normal S. aureus: at a concentration of 3±4 mg/L, all the cells of normal S. aureus are inhibited, even with a high inoculum of 107 108 CFU [2,20]. In contrast, all the GISA will yield at least 1000 colonies, sometimes many more. The number of colonies will decrease with increased vancomycin concentrations, up to 10± 32 mg/L. As suggested by Hiramatsu, a `onepoint PAP', which is obtained by screening a high inoculum on plates containing 4±6 mg/L of vancomycin, clearly discriminates between GISA and normal S. aureus. Because of the poor reproducibility of the pro®les in different laboratories, the poor stability of the resistance, and the lack of
standardized procedures, PAP remains the only simple and reliable technique for detecting GISA strains. The PAP also very clearly explains the relationship between the inoculum size and the MIC, which depends on the heterogeneity of the strain. With a light inoculum of 102 103 CFU/spot, as often performed on agar, the MICs will decrease to fully susceptible levels. The main question is: are GISA really responsible for an increased clinical failure rate? If so, are there other therapeutic options? Failure of vancomycin in the treatment of staphylococcal infections has been observed and reported over the last 40 years. As with other antibiotics, these failures can be explained not only by decreased susceptibility, but also by low antibiotic serum levels, undrained abscesses, or the presence of foreign bodies. All the failures reported so far for vancomycin in the treatment of GISA infections were `expected failures', in that there were several `good' reasons for them, as reported for many other antibiotics, in addition to vancomycin: 1. Inadequate vancomycin levels or no monitoring. 2. Use of teicoplanin, which is less active, or no monitoring. 3. Antagonistic combinations with aminoglycosides and/or b-lactams. 4. No drainage of large pus collections. 5. Foreign devices not removed. 6. Very severely ill patients. In some patients, several explanations were relevant. 1. What is an adequate serum level for vancomycin? In a paper published in 1993, it was considered that at least 43% of patients treated with vancomycin had inappropriate drug monitoring. It was a very optimistic approach, because `acceptable serum levels were a trough of 5±10 mg/mL and a peak of 20±25 mg/mL'. In contrast to many papers suggesting that it is not necessary to perform vancomycin serum assays, we perform these routinely for all severely ill patients. Patients are treated according to well established nomograms with the standard dose of 1 g twice daily or by continuous infusion after a 1-g loading dose. Vancomycin has a very variable half-life, 3±12 h in healthy volunteers, and there is no reason for it
ß 2003 Copyright by the European Society of Clinical Microbiology and Infectious Diseases, CMI, 9, 761±765
Editorial 763
to have a smaller range in severely ill patients. Assuming 50% protein binding in serum and increased non-speci®c `af®nity trapping' by the staphylococcus, the effective serum concentration above the MIC for 40±50% of the dosing interval or, better, above the concentration inhibiting 100% of cells with a high inoculum (as determined by PAP) should be at least three times this concentration: (a) 9±12 mg/L for a fully susceptible strain (MIC: 1±2 mg/L with 100% inhibition at 3±4 mg/L). (b) 30 mg/L for a hetero-GISA type Mu 3 (MIC 2± 3 mg/L with 100% inhibition at 10 mg/L). (c) At least 40 mg/L for Mu 50-like strains, which is more dif®cult to maintain safely, and without exceeding 50 mg/L. This may represent 4± 5 g/daily administered as a continuous infusion (N. Desplaces, personal communication). It should be pointed out that puri®ed vancomycin is very well tolerated and has little resemblance to the historical `Mississipi mud' used 45 years ago. According to the results obtained in 553 patients in our hospital receiving vancomycin on the basis of published nomograms, we consider that only 6.5% and 23% of patients receiving vancomycin either as a standard 1-g treatment twice daily or continuous infusion (after a 1-g loading dose), respectively, had adequate levels for the treatment of Mu 3-type GISA strains; more than 40% of patients treated with this standard dose had inadequate levels for treatment of even fully susceptible strains. Among all the published failures, only one patient [12] had adequate serum levels; the others occasionally had excessively low trough and/or peak serum levels. In conclusion, the only thing as dangerous as an overdose is an underdose. 2. Teicoplanin selects ®rst-step mutants or mutants with higher MICs much more easily and has been associated with clinical failures. Moreover, with 90% protein binding and in the absence of a loading dose for three days and monitoring, effective serum levels will remain very low, as observed in one patient [14]. In many other patients [13,14,17], teicoplanin, and not vancomycin, has eventually selected GISA. 3. Vancomycin or teicoplanin has often been combined with an aminoglycoside for gentamicin-resistant strains [2,10,11,14,17]. We and many
others have reported for more than 20 years that gentamicin-resistant S. aureus produces the bifunctional enzyme aminoglycoside phosphotransferase (APH) (200 ) aminoglycoside acetyltransferase (AAC) (60 ) which inactivates all aminoglycosides (not streptomycin, an aminocyclitol). As a consequence, all combinations with b-lactams or a glycopeptide are antagonistic. The already weak bactericidal activity of vancomycin will be reduced. In respect of the combinations of vancomycin with b-lactams, some authors have reported that this combination is synergistic [6,9,14]. This is true for some b-lactam concentrations, or rather when the combinations are improperly tested. As already suggested by Hiramatsu, with the exception of Mu 50, combinations of b-lactams with vancomycin are antagonistic, at least over a range of concentrations. This was the case for a few patients [10,12]. 4. Because of the non-speci®c af®nity trapping, vancomycin is less effective in the presence of a high inoculum; this is particularly true for GISA, and this was the reason for the failures in patients [2,12,13,17] who had undrained abscesses. For the same reasons, some patients were cured after drainage [13,17], whereas others died [12,13]. 5. As with other antibiotics, the presence of catheters and other foreign devices strongly reduces the activity of vancomycin, especially against GISA, which have a thicker cell wall and increased attachment to arti®cial devices. These devices should be removed as soon as possible. Removal of catheters has undoubtedly contributed to the recovery of many patients [13,17,28]. The presence of a catheter for a long period is responsible for thrombophlebitis and recurrent breakthrough bacteremia; this was the reason for failure in several patients [9±11,15,17]. 6. One patient died within 12 h of admission; no antibiotic could have helped him [10]. Another patient died because he refused surgery [29]: it is well known that about 50% of leftsided staphylococcal endocarditis patients will die in the absence of surgery. Several patients were very severely ill, old and at a late stage of a progressive disease; it is very unlikely that other antibiotics would have been of any help [9,14,15,29]. It has not yet been demonstrated that vancomycin, administered at high dosage and properly
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764 Clinical Microbiology and Infection, Volume 9 Number 8, August 2003 monitored, is not effective against GISA, at least of the Mu 3 type. Assuming than vancomycin alone would not be effective, what are the other issues? 1. Vancomycin in combination with b-lactams: these associations are antagonistic as suggested by Hiramatsu, b-lactams may increase the MIC of vancomycin or select for heterogeneous strains [20]. 2. Vancomycin in combination with other antibiotics. The association of vancomycin and aminoglycosides is antagonistic for all staphylococci producing aminoglycoside-inactivating enzymes. Against susceptible bacteria, the association is highly bactericidal with other antibiotics (quinupristin±dalfopristin, rifampin, fusidic acid, cotrimoxazole), the association is indifferent or displays an antagonistic effect for a particular concentration range. (R. Atoui, unpublished data). 3. Other antibiotics without glycopeptides: most of the GISA are susceptible to linezolid, quinupristin±dalfopristin, chloramphenicol or co-trimoxazole, and occasionally to tetracyclines, fusidic acid or rifampin. These antibiotics have been successfully used for the treatment of a few patients [11,14,17]. 4. Investigational compounds: tigecycline, oritavancin or daptomycin have not yet been evaluated in vivo. Finally, all the new ¯uoroquinolones have poor activity against these multiresistant bacteria and should not be used. There are no well documented clinical studies assessing the role of decreased vancomycin susceptibility as the sole or main factor in therapeutic failure. In all the reported failures, there have been several important confounding factors explaining a breakthrough infection. Obviously, however, there is no reason for GISA to give better clinical results than fully susceptible strains or to cure the patient of a fatal underlying disease. It is dif®cult to predict the future. Once staphylococci acquire resistance to older and newer (linezolid, quinupristin±dalfopristin) antibiotics in addition to true high-level resistance to vancomycin (vanA), as just described during the preparation of this manuscript [30], then `apocalypse now' may become true. REFERENCES 1. Tabaqchali S. Vancomycin-resistant Staphylococcus aureus: apocalypse now ? Lancet 1997; 350: 1644±45.
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